Singularity

Harry and Barry are twins, temporarily staying at their recently-deceased great-uncle's farm when they discover something strange about the backyard shed. 

Harry is content being a twin, he's more bothered by how vocally Barry seems to hate being his twin. This tension drives a very drastic choice Harry makes late in the book. Their relationship revolves around meager attempts at bonding followed by bullying in various forms. There's a neighbor girl who exists mostly as an outsider to verify that what they think is happening has actually happened. She serves to shift the balance of power between them (usually in Barry's favor) rather than having an endless back-and-forth of the kind it seems they had at home.

The worldbuilding is well done, a bunch of little things early on end up having a pretty cool answer, fitting together in a great way. I like the plot, but the ending has an ambiguity which makes it hard to know whether it's supposed to be read triumphantly or as horror. I come down more on the side of horror, but I definitely didn't pick up on that when I first read it as a teenager.